Heretofore, emulsions prepared through emulsion polymerization of (meth)acrylate monomers are widely used in various fields of coating materials, paper processing, fiber processing, etc. In emulsion polymerization of these monomers, generally used is an anionic or nonionic surfactant as a stabilizer from the viewpoint of the stability of the emulsion polymerization. However, the emulsion that uses such a surfactant as a stabilizer is defective in that its mechanical stability is poor, and therefore it could not be used in the field that requires high mechanical stability, for example, for admixtures such as cement and mortar.
For the purpose of solving the problem as above, proposed is a method which comprises using polyvinyl alcohol (PVA) having a degree of polymerization of at most 500, preferably at most 300, or comprises emulsion polymerization in the presence of PVA and a chain transfer agent (Patent Reference 1, Patent Reference 2) to thereby improve the mechanical stability of the emulsion. However, using such PVA is still defective in that PVA could not sufficiently exhibit its property as a protective colloid and could not completely satisfy the mechanical stability of the emulsion, and, in addition, the strength of the emulsion film is poor. Also proposed is a method of using a mercapto group-having PVA-type polymer as an emulsion dispersion stabilizer (Patent Reference 3, Patent Reference 4, Patent Reference 5). However, this is problematic in that, when an ordinary initiator such as potassium persulfate, ammonium persulfate or hydrogen peroxide alone, or a redox initiator comprising a combination of such peroxides with various reducing agents is used, then the grafting efficiency on the PVA-type polymer is low and sufficiently practicable stability is difficult to ensure. On the other hand, when an initiator such as potassium bromate capable of generating a radical only through redox reaction with the mercapto group of the PVA-type polymer is used, then the polymerization stability may be improved but the method with it is still problematic in that the reaction reaches dead-end at the time when the mercapto group of the PVA-type polymer is exhausted and therefore the polymerization is difficult to control and complete. A method is disclosed for producing an emulsion by adding a polyvinyl alcohol during a stage after the start of polymerization and before the start of aging (Patent Reference 6). However, since the method uses an emulsifier at the start of emulsion polymerization, it is problematic in that, when the emulsion is used in various applications, the emulsifier may migrate to have negative influences on the physical properties of the emulsion products Further, as is obvious from Comparative Examples 16 and 17 given hereinunder, the emulsion produced according to the method does not contain particles having a sharp particle size distribution of at least 0.3 in terms of the “factor a” as defined herein though its mechanical stability is good, and, in addition, the film strength and the transparency of the emulsion are not satisfactorily good.
Another polymerization method is proposed which comprises continuously or intermittently adding a monomer such as (meth)acrylate monomer or diene monomer and a water-soluble polymer protective colloid to a reactor, and this improves the mechanical stability of the resulting polymer emulsion (Patent Reference 7). However, the method is defective in that the polymerization operability thereof is not good and, in addition, the reproducibility thereof to give emulsion is not also good since the heterogeneous emulsion polymerization is significantly influenced by various factors such as the shape of the stirring blade, the stirring speed and the polymerization scale (the capacity of the polymerization reactor).
Still another polymerization method is proposed which comprises polymerizing an acylate monomer in the presence of PVA in a mode of emulsion dispersion to a particle size of at most 0.5 μm (Patent Reference 8), and this improves the polymerization stability therein. However, the method indispensably requires a forced emulsification device such as homomixer and requires a severe polymerization condition of controlling the oxygen concentration in the aqueous phase during the polymerization to at most 0.3 ppm, and its general use is difficult. Further, as is obvious from Comparative Examples 14 and 15 given hereinunder, the emulsion produced according to the method does not contain particles having a sharp particle size distribution of at least 0.3 in terms of the “factor a” as defined herein though its mechanical stability is good, and, in addition, the film strength and the transparency of the emulsion are not satisfactorily good.
Example 2 of Patent Reference 9 proposes emulsion polymerization that comprises feeding an acrylate (small amount), a peroxide (small amount), PVA and an iron compound into a reactor in the initial stage of polymerization followed by successively adding thereto an acylate (large amount), a peroxide and a reducing agent (Rongalite); Example 3 of Patent Reference 9 proposes emulsion polymerization that comprises feeding an acrylate (total amount), a peroxide (total amount), PVA and an iron compound into a reactor in the initial stage of polymerization followed by successively adding a reducing agent (Rongalite) thereto. PVA used in Patent Reference 9 is a low-polymerization-degree PVA having a molecular weight of from 5000 to 13000 (about 100 to 300 in terms of the degree of polymerization), or a high-saponification-degree PVA having a degree of saponification of at least 96.5 mol %. However, as is obvious from Comparative Examples 10 to 12 and Comparative Examples 18 and 19 given hereinunder, the emulsion produced according to the method proposed in Patent Reference 9 has poor film strength and does not contain particles having a sharp particle size distribution of at least 0.3 in terms of the “factor a” as defined herein, and, in addition, the transparency of the emulsion film is not satisfactorily good.
As described above, various proposals have heretofore been made of (meth)acrylate resin emulsions that comprise PVA-type polymer as a protective colloid. At present, however, no one knows those that completely satisfy the emulsion polymerization stability and the polymerization operability thereof and that are excellent in all the film strength, the film transparency and the mechanical stability thereof and also in the alkali resistance thereof and are therefore applicable to general use.
Patent Reference 1:
JP-A 4-185606 (claims)
Patent Reference 2:
JP-A 4-185607 (claims)
Patent Reference 3:
JP-A 60-197229 (claims)
Patent Reference 4:
JP-A 6-128443 (claims)
Patent Reference 5:
JP-A 7-278212 (claims)
Patent Reference 6:
JP-A 8-245706 (claims, Example 1 and Example 3)
Patent Reference 7:
JP-A 11-335490 (claims)
Patent Reference 8:
JP-A 2000-256424 (claims, Example 1 and Example 3)
Patent Reference 9:
JP-A 10-60055 (claims, Example 2 and Example 3)